With a brake-by-wire, or similar type of vehicle braking system, where the application of fluid brake pressure to the wheel brakes can be generated by an electrically or an electro-hydraulically controlled means rather than by the manual application of force to the master cylinder, the master cylinder is typically isolated from the rest of the braking system during brake operation. This changes the response characteristics at the vehicle's brake pedal from those conventionally provided and can prevent the driver from experiencing the customary brake pedal travel and feel. These automated types of braking systems generally do not include a conventional booster between the brake pedal and the master cylinder, which may contribute to pedal response changes.
It is preferable that the vehicle driver experience a conventional pedal response when applying a vehicle's brakes on a vehicle with electric or electro-hydraulic brake control. In some applications, such a control system is preferably "transparent" to the driver, and the brake apply system would be perceived to respond the same as a conventionally boosted apply system. Accordingly, brake by wire systems include ancillary components to approximate conventional pedal characteristics during brake application. These ancillary components increase costs, require additional space and can interject discernible differences into the pedal feel characteristic during brake application cycles.
A known device that mimics the conventional pedal feel when the master cylinder is isolated from the remainder of the braking system is disclosed in U.S. Pat. No. 5,246,283 entitled Electro-Hydraulic Brake Apply System, which issued Sep. 21, 1993. That patent describes a stand alone accumulator with movable pistons separated by a pair of springs. The pistons move against the springs to provide compliance in the system, so that the driver experiences customary pedal feel and travel rather than a hard pedal.
Additional efforts have been directed at more closely approximating conventional brake apply feel at the pedal. The use of additional springs, providing gaps adjacent springs or integrating the device into a master cylinder have been found to help in this regard. One such device is disclosed in U.S. Pat. No. 5,630,217 entitled Compliant Mater Cylinder, which issued Feb. 18, 1997. That patent discloses a master cylinder with a compliant assembly that provides conventional pedal feel and travel characteristics when the master cylinder pistons are substantially immovable. The compliant assembly includes a push rod piston that is movable against a selected amount of fluid pressure and spring force.
The strides that have been made in approximating the conventional pedal feel characteristics of a brake apply system have resulted in good emulation of pedal travel versus pedal force for a typical passenger vehicle during various brake applications. Accordingly, automated apply systems are presently capable of providing acceptable pedal feel characteristics. However, because pedal feel is a subjective quantity, additional advancements in the art are still preferable, especially since a great many variables contribute to the pedal feel response of any given system.